High density interconnect (HDI) printed circuit boards (PCBs) contain multiple layers of copper interconnects separated by insulating material that are united by metallized features such as through-holes and vias. The most common method for the metallization of the through-holes and vias is electroless copper. Most catalysts for electroless copper are based on either colloidal or ionic palladium. In the activation process, the palladium-based colloid is adsorbed onto an insulating substrate such as epoxy or polyimide to activate electroless copper deposition. Theoretically, for electroless metal deposition, the catalyst particles play roles as carriers in the path of transfer of electrons from reducing agent to metal ions in the plating bath. Although the performance of an electroless copper process is influenced by many factors such as composition of the plating solution and choice of ligand for copper, the activation step is the key factor for controlling the rate and mechanism of electroless metal deposition. Palladium/tin colloids have been commercially used as an activator for electroless metal deposition for decades, and its structure has been extensively studied. The colloid includes a metallic palladium core surrounded by a stabilizing layer of tin(II) ions. A shell of [SnCl3]− complexes act as surface stabilizing groups to avoid agglomeration of colloids in suspension. Yet, its sensitivity to air and high cost leave room for improvement or substitution.
While the colloidal palladium catalyst has given good service, it has many shortcomings which are becoming more and more pronounced as the quality of manufactured printed circuit boards increases. In recent years, along with the reduction in sizes and an increase in performance of electronic devices, the packaging density of electronic circuits has become higher and subsequently required to be defect free after electroless plating. As a result of greater demands on reliability alternative catalyst compositions are required. The stability of the colloidal palladium catalyst is also a concern. As mentioned above, the palladium/tin colloid is stabilized by a layer of tin(II) ions and its counter-ions can prevent palladium from aggregating. The tin(II) ions easily oxidize to tin(IV) and thus the colloid cannot maintain its colloidal structure. This oxidation is promoted by increases in temperature and agitation. If the tin(II) concentration is allowed to fall close to zero, then palladium particles can grow in size, agglomerate, and precipitate.
Ionic catalysts have several advantages over the colloidal catalysts currently employed. First, ionic catalysts are more resistant toward oxidizing environments due to the absence of tin(II) ions and because the catalyst ions are already in an oxidized state. Additionally, ionic catalysts can penetrate deep into every recess of a substrate leading to uniform coverage of rough features. Lastly, ionic complexes deposit less catalyst material, and as such, provide the reduced residual conductivity necessary for fine line technology and lower catalyst consumption.
Ionic silver catalysts would be advantageous to use because of the much lower cost of silver relative to palladium. However, in contrast to palladium, silver catalysts suffer from low catalyst activity: longer plating initiation times and slower electroless deposition rates; and ionic silver rapidly immersion plates on copper leading to interconnect defects. The most commonly encountered form of ionic silver catalysts are based on tin(II)/silver(I) activation. In these systems, a substrate is first activated with strong acid followed by tin(II) and then silver(I). Widespread adoption of tin(II)/silver(I) catalyst systems have been thwarted by immersion plating, the strongly acidic etches necessary for tin(II) adsorption and industrial trends favoring alternatives to tin(II). Accordingly, there is a need for an improved method of electroless plating metal with ionic silver catalysts.